Wireless communication systems are assuming ever-increasing importance for the transmission of data, which is to be understood in its largest sense as covering speech or other sounds and images, for example, as well as abstract digital signals.
Currently proposed standards for wireless communication systems include the 3GPP (3rd generation Partnership Project) and 3GPP2 standards, which use Code Division Multiple Access (‘CDMA’) and Frequency Division Duplex (‘FDD’) or Time Division Duplex (‘TDD’), the HIPERLAN and HIPERLAN2 local area network standards of the European Telecommunications Standards Institute (‘ETSI’), which use Time Division Duplex (‘TDD’) and the International Telecommunications Union (‘ITU’) IMT-2000 standards. The present invention is applicable to systems of these kinds and other wireless communication systems.
In order to improve the communication capacity of the systems while reducing the sensitivity of the systems to noise and interference and limiting the power of the transmissions, various techniques are used separately or in combination, including space-time diversity, where the same data is transmitted over different transmit and/or receive antenna elements, and frequency spreading, such as Orthogonal Frequency Division Multiplex (‘OFDM’) where the same data is spread over different channels distinguished by their sub-carrier frequency.
At the receiver, the detection of the symbols is performed utilising knowledge of the complex channel attenuation and phase shifts: the Channel State Information (‘CSI’). The Channel State Information is obtained at the receiver by measuring the value of pilot signals transmitted together with the data from the transmitter. The knowledge of the channel enables the received signals to be processed jointly according to the Maximum Ratio Combining technique, in which the received signal is multiplied by the Hermitian transpose of the estimated channel transfer matrix.
Two broad ways of managing the transmit diversity have been categorised as ‘closed loop’ and ‘open loop’. In closed loop signal transmission, information concerning the transmission channels is utilised at the transmitter to improve the communication. For example, the document Tdoc SMG2 UMTS-L1318/98 presented to the ETSI UMTS Physical Layer Expert Group describes operation of a Transmit Adaptive Array (TxAA) FDD scheme in which the dedicated channels are transmitted coherently with the same data and code at each transmit antenna, but with antenna-specific amplitude and phase weighting. The receiver uses pilots transmitted on the Common Channels to separately estimate the channels seen from each antenna. The receiver estimates the weights that should be applied at the transmitter to maximise the power received at the receiver, quantises the weights and feeds them back to the transmitter. The transmitter applies the respective quantised weights to the amplitudes and phases of the signals transmitted from each transmit antenna of the array. U.S. Pat. No. 6,192,256 assigned to the assignee of the present invention describes a closed loop transmission system of this kind. Alternatively, in TDD systems, the channel state information for weighting the signals applied to the downlink transmit antennas may be derived from the uplink signals, assuming that the downlink and uplink channels are reciprocal, without transmission of any specific channel or weighting information from the receiver to the transmitter.
Further improvement in communication may be obtained by use of a RAKE receiver. In a multi-path channel, the original transmitted signal reflects from obstacles such as buildings, and mountains, and the receiver receives several copies of the signal with different delays. If the signals arrive more than one elementary signal element apart from each other, a simple receiver can resolve them. Actually, from each individual multi-path signal's point of view, other multi-path signals can be regarded as interference and they are suppressed by the processing gain of a simple receiver or a single RAKE receiver finger.
A RAKE receiver obtains further benefit by combining the resolved multi-path Signals. The Review “An overview of CDMA evolution toward wideband CDMA” by Ramjee Prasad and Tero Ojanperä, published by IEEE Communications Surveys describes an example of a RAKE receiver. After spreading and modulation the signal is transmitted and the signals in the multi-path channels are delayed and attenuated by respective amounts. The RAKE receiver has a plurality of receiver fingers for receiving different multi-path components of the signal. In each finger, the received signal is correlated by a spreading code, which is time-aligned with the respective measured delay of the multi-path signal. After de-spreading, the signals are weighted and combined, for example by maximal ratio combining, that is to say weighting each signal by the path gain (attenuation factor). Since the received multi-path signals fade independently, diversity order and thus performance are improved.
In practice, movement of a mobile receiver will change the scattering environment and thus the delays and attenuation factors will change as well. The RAKE receiver fingers may be defined by software algorithms rather than by hardware. The transmission multi-path channel profile is measured and the RAKE fingers may then be reallocated. Small-scale changes are taken care of by a code-tracking loop, which tracks the time delay of each multi-path signal.
The document “Transmit diversity with joint pre-distortion”, Tdoc 3GPP TSGR1#6(99)918, August 1999 presented to the 3GPP working group 1, proposes for UMTS TDD mode pre-distorting the transmit signals separately (or simultaneously) on each smart antenna element in order to remove the need for joint detection at the receiver: the objective is stated to be to be able to use a single-finger RAKE receiver, that is to say that the transmitted signal is modified so that the received signal appears to the receiver as if it was not a multi-path signal but a single-path signal. No advantage is gained from the multi-path diversity.
The article “Pre-equalization for MIMO wireless channels with delay spread” by H. Sampath, H. Bölcskei, A. J. Paulraj, published by IEEE in VTC 2000 describes an OFDM transmission system in which channel knowledge made available at the transmitter is used to pre-equalise the signals transmitted from the transmit antennas, so as to reduce the complexity of the mobile station. The system includes a Finite Impulse Response (FIR) filter that combines copies of the transmit signals with respective delays and weights (gains) and launches the combined signals from the transmit antennas.
In both case, such schemes attempt to make the channel look flat in order to minimize the interference and avoid the use of a multi-finger RAKE receiver or equaliser at the receiver. The present invention obtains improved performance compared to these systems by exploiting the multi-path signals.